This invention relates to pH sensors for measuring pH of a solution, and particularly to minimizing cross-talk between currents carried by electrodes of a pH sensor.
Sensors that measure ion content, also known as pH sensors, are used in industrial process control systems to measure the hydrogen (H+) or hydroxyl (OHxe2x88x92) ion content, or pH, of a solution of the industrial process. pH sensors employ at least two electrodes, an ion-specific electrode (commonly called a glass electrode due to its construction) and a reference electrode. A pH analyzer operates the pH sensor by measuring a voltage across both the ion-specific electrode (IGLASS) and the reference electrode (IREF). Thus, the voltage between a common potential, such as electrical ground, and each of the ion-specific electrode (VGLASS) and the reference electrode (VREF) is measured, and the difference between the two voltages (VGLASSxe2x88x92VREF) represents the pH value. The sensor is calibrated so that there is a known relationship between VGLASS and VREF when the sensor is in a neutral (pH=7.0) solution.
The impedances of the ion-specific and reference electrodes are used for diagnostic and maintenance purposes. Thus, if a sensor becomes cracked or otherwise deteriorates, the ability of the sensor to accurately measure the ion content of the solution also deteriorates. Electrode deterioration is determined from the impedance (resistance) of the electrodes. If the impedance of one electrode changes, the sensor may require re-calibration or replacement.
The impedance of the ion-specific electrode may be significantly greater than that of the reference electrode; the ion-specific electrode often exhibiting as much as 10,000 times the impedance of the reference electrode. Consequently, the current to the reference electrode may be significantly greater than that to the ion-specific electrode; IREF often being as much as 10,000 times IGLASS. With both IGLASS and IREF flowing through the solution at the same time, cross-talk between the currents in the ion-specific and reference electrodes may alter the current flows in the solution, and hence the voltage outputs, resulting in error in impedance measurements of the electrodes. The present invention is directed to a method of multiplexing IGLASS and IREF to minimize cross-talk to improve impedance measurements of the electrodes.
According to the present invention, cross-talk between the reference and ion-specific electrodes is minimized by applying the first current, IGLASS, to the ion-specific electrode during a first time period and applying the second current, IREF, to the reference electrode a second time period.
In a preferred form of the invention, the first and second currents are substantially direct (DC) currents and application of the first and second currents to their respective electrodes is interleaved so that the first current is applied to the ion-specific electrode while the second current is off and the second current is applied to the reference electrode while the first current is off.
In another preferred form of the invention, a current spike is added to the second current whenever the second current changes to off. The current spike has a value to discharge any charge in the solution due to the second current.
In another preferred form of the invention, the ion-specific electrode and the reference electrode, together with a common electrode, are applied to a solution. A difference between voltages at the ion-specific electrode, VGLASS, and the reference electrode, VREF, each relative to the common electrode, is a measure of pH of the solution.
In another preferred form of the invention, the impedance of the ion-specific electrode is calculated based on the first current, IGLASS, and the voltage measured between the ion-specific electrode and the common electrode.